1. Field of the Invention
The present invention relates to operational amplifiers, and more particularly, to operational amplifiers with increased common mode input range.
2. Related Art
Standard NTSC color video systems have been commonplace since 1970 and continue to be used widely today. The baseband NTSC video signal is an analog signal with an amplitude of approximately 1.3 Volt-peak-to-peak (Vpp) and has remained largely unchanged since the technology's inception. Early video systems had relatively large supply voltages where the difference in the positive and negative supply voltage would often be 30 volts. Processing a 1.3 Vpp signal using analog circuits with 30 V across the supplies means that issues of headroom were rarely a problem. Problems with headroom occur when the input voltage comes too close to the circuit's power supply voltages such that the circuits cannot operate properly. Given a specific supply voltage, a common-mode input range (CMIR) is defined as the range of input voltages over which the circuit can operate correctly.
The desire to integrate many circuits, both analog and digital, onto a single IC means using CMOS technologies with very small geometries. As transistor sizes shrink, more circuits can be integrated using the same amount of silicon area. However, as the transistor size shrinks, so does the maximum voltage across which the devices can safely operate. As the supply voltage approaches the signal amplitude, the challenges in circuit design increase dramatically. The required CMIR may include much of the available supply voltage. Attenuation of the NTSC signal is usually undesirable, because the NTSC signal is single-ended, and such an attenuation will result in a serious noise problem.
Many operational amplifiers (op amps) use rail-to-rail circuit techniques which allows the CMIR to include the entire supply voltage. These topologies often employ two input stages, one for operation near each supply voltage. One input stage will use a PMOS differential pair and the other will use a NMOS differential pair. Because the transconductances of these two input pairs are not matched and will not track each other over process variations, the linearity of the overall amplifier is degraded, and high performance is difficult to achieve.
Another op amp topology often chosen for it's high CMIR is the folded-cascode topology (See “Analysis and Design of Analog Integrated Circuits”, Gray, Hurst, Lewis & Meyer, John Wiley and Sons, 4th ed. 2001, pp. 446-450). Defining the MOS threshold voltage as Vt and the overdrive voltage VGT=VGS−Vt, in FIG. 6.28 of Gray et al., maximum input common-mode voltage VCMI(max)=VDD−Vt5−VGT5−VGT1 (assume matched transistor pairs M1-M2, M11-M12, M1A-M2A in FIG. 6.28 of Grey et al., with the numeric subscript referring to the transistor number). Also the Vt's and VGT's are assumed to be positive whether the transistor is NMOS or PMOS. Voltages greater than VCMI(max) will cause M5 to leave saturation and it's current will drop. The folded-cascode circuit often allows the VCMI to reach the negative supply, usually ground in low supply voltage circuits, without any problems. However, in unity gain buffer configurations, where the inverting op amp input is tied to the output, it is the output which will limit the voltage swing.
Although the linearity of the folded-cascode op amp is better than the typical rail-to-rail designs, it still has linearity problems due to the finite output impedance of M5 in FIG. 6.28 of Gray et al. As the common mode input voltage VCMI changes, the tail current ID5 will change, which will in turn change the gain of the stage. The stage gain varies as a function of the input stage transconductance gm times the output resistance R0. The gain goes down as the tail current increases. To overcome this problem, the tail current source could be cascoded, however this would further reduce VCMI(max) by an additional VGT term.